1. Field of the Invention
This invention relates to holographic optical elements and methods of forming such elements in such a manner as to make them free of unwanted secondary fringes by use of light beams having short coherence path lengths.
2. Prior Art
Holographic optical elements (HOEs) comprise plates having fine fringe-like structures which are formed by photographic processes involving the recording of interference fringe patterns of two mutually coherent light beams. In use, the fringe-like structures diffract light wavefronts incident on the HOEs to act as lenses, filters and the like.
When HOEs are formed by illuminating a photographic emulsion coated on a supporting plate with a pair of coherent light beams to form an interference pattern, it is difficult to prevent various secondary reflections of the incident light beam from interfering to produce unwanted, secondary fringe patterns of a lower intensity than the primary fringes. One source of these secondary reflections is the reflection that occurs when the primary interfering beam, after having passed through the photographic emulsion, enters the immediately contacting media, be it air, a liquid or solid material. Despite the fact that the contacting media is transparent, because of the differences in indices of refraction between the emulsion and this media, some reflections back into the emulsion will occur from the interfaces. These reflections can be minimized using index-matching techniques and anti-reflection coatings, but cannot be completely eliminated. If the primary light beam has sufficient coherence path length, these reflections back into the emulsion will interfere with the primary beam to produce secondary fringes. In many applications of HOEs, the resulting weak, secondary fringe systems will not deleteriously affect the performance of the device, but in certain applications such secondary fringes are highly undesirable. For example, holographic optical elements are used as combiners in head-up display systems in which they act to superimpose instrument displays on the pilot's view through the aircraft windshield. In this application secondary fringe systems may produce undesirable images to the pilot which may interfere with his perception of both the instrument displays and the view through the windshield.